Modeling and simulation of a photovoltaic thermal-compound thermoelectric ventilator system

Applied Energy 228 (2018) 1887–1900

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Modeling and simulation of a photovoltaic thermal-compound thermoelectric ventilator system ⁎

T

⁎

Zhongbing Liua, Yelin Zhanga, Ling Zhanga, , Yongqiang Luoa, , Zhenghong Wua, Jing Wua, Yingde Yinb, Guoqing Houc a

College of Civil Engineering, Hunan University, Changsha 410082, PR China Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, PR China c China Machinery International Engineering Design & Research Institute Co., Ltd, PR China b

H I GH L IG H T S

model is established for PVT-TEV system. • AThedynamic system heat fresh air by PVT coupled with thermoelectric ventilator system. • Sensitivity is implemented for model parameters optimization. • The averageanalysis COP of the PVT-TEV system can reach 13.85. •

A R T I C LE I N FO

A B S T R A C T

Keywords: Solar energy Photovoltaic thermal (PVT) Thermoelectric Energy recovery COP

This paper presents a dynamic model for a photovoltaic thermal-compound thermoelectric ventilator (PVT-TEV) system, in which photovoltaic thermal (PVT) collector generates electricity in winter and simultaneously preheats the fresh air to achieve comprehensive utilization of solar energy. The preheated fresh air is further heated by the thermoelectric ventilator (TEV) and then pumped into the indoor room. The PVT-TEV system has the function of the sunshade, power generation, waste heat recovery and fresh air supply for buildings. The model is validated by the data collected from PVT-TEV system under real climate conditions. The results show that the simulated value is in good agreement with the experimental value. The performance of the PVT-TEV system under different working condition is also analyzed by using the model. Increasing the fresh air mass flow rate can improve the electrical and heating performance of PVT-TEV system. However, the total coefficient of performance (COPtotal) of PVT-TEV system does not grow when the fresh airflow rate rises from 93 m3/h to 123 m3/h and 153 m3/h due to the increase of fan power. The heat gain of the TEV system and the fresh air outlet temperature of the PVT-TEV increase with the working current. The growth of indoor temperature can improve the performance of PVT-TEV system. The model of PVT-TEV system proposed in this paper provides a foundation for the structural design and annual performance optimization of PVT-TEV system.

1. Introduction As global warming and energy crisis intensify, energy conservation has become the focus of global concern. Buildings are the world’s largest consumer of energy. About 35–40% of total energy is consumed by buildings in developed countries [1]. In China, buildings consume about 30% of energy and it is predicted to continue to increase in future [2]. Heating, ventilation and air-conditioning (HVAC) systems in buildings consume a significant amount of energy [3,4]. How to minimize need for energy use in buildings through more energy-efficient

⁎

measures and adopt renewable energy to meet the minimal energy needs [5–7], and improve indoor quality has become a common concern for researchers [8]. As an essential system in buildings, the fresh air system consumes about 20–40% of the overall HVAC system’s energy [9]. Therefore, reducing the energy consumption of building fresh air is of great significance to building energy conservation. External shading devices can prevent direct solar radiation into the building in the summer period, which have been widely used in buildings to improve the thermal and lighting environment [10,11]. External shading devices could reduce solar radiation into the building

Corresponding authors. E-mail addresses: [email protected] (Z. Liu), [email protected] (L. Zhang), [email protected] (Y. Luo).

https://doi.org/10.1016/j.apenergy.2018.07.006 Received 23 April 2018; Received in revised form 18 June 2018; Accepted 1 July 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.

Applied Energy 228 (2018) 1887–1900

Z. Liu et al.

Nomenclature

k K Kl l lduct lTEV ṁ n n0 n1 Np Ns Nu Pfan PPV PTE Pr q Qc Qh QPV r Rc

Abbreviations COP DE HVAC MPPT PV PVT PVT-TEV STC TE TEM TEV ZT

coefficient of performance differential evolution heating, ventilation and air-conditioning Maximum Power Point Tracking photovoltaic photovoltaic thermal photovoltaic thermal-compound thermoelectric ventilator standard testing condition thermoelectric thermoelectric modules thermoelectric ventilator thermoelectric figure of merit

Symbols a Af Afin Ai Aln APV Atotal Aw Azf b c COPh COPtotal d0 d1 do eP ePV Eg G H ho hco hci hif hin HTEV,low HTEV,up hzf i I I0 Im Iph IPV Isc ITEV j1

thermoelectric material seebeck coefficient (V/K) total surface area of the heat sink (m2) effective heat transfer area of the fins (m2) heat transfer area of the node region (m2) condensing section surface area of the inner wall of the heat pipe (m2) area of PV panel (m2) actual total area of the fins on both sides (m2) heat pipe condensation section diameter area (m2) surface area of the inner wall of the heat pipe in the evaporation section (m2) thickness of the air duct (m) specific heat capacity (J/(kg K)) coefficient of performance of the TEV in heating mode (–) total coefficient of performance (–) inner diameter of the air duct (m) outer diameter of the air duct (m) heat pipe diameter (m) thickness of the node region of the insulation board (m) thickness of the PV panel (m) energy band gap (eV) intensity of solar radiation (W/m2) width of the fin (m) convection coefficient between indoor air and thermal insulation materials (W/m2 K) convection coefficient between the air and both the outside of PV panel and insulation board (W/m2 K) convection coefficient between the fresh air and both the PV panel and insulation board (W/m2 K) heat transfer coefficient between air and heat sink (W/m2 K) heat transfer coefficient between the inner wall and the working fluid of the evaporation section (W/m2 K) height of the lower channel of the TEV (m) height of the upper channel of the TEV (m) heat transfer coefficient between the inner wall and the working fluid of the evaporation section (W/m2 K) node counter (–) working current of TEV system (A) diode saturation current (A) current at MPP (A) photo current (A) working current of PVT system (A) short circuit current of the PV module (A) total working current of TE modules (A) time counter (–)

Re Rh Rp RPV Rs Rtotal s Toc Toh TPVO Tw U v V va Va vm Vm Voc VPV Vth w wTEV x X y z

thermal conductivity of thermoelectric material (W/K) Boltzmann’s constant (J/K) temperature coefficient of the short circuit current (V/K) length of the PVT system (m) length of the air duct (m) length of the TEV (m) mass flow rate of fresh air (kg/s) number of node regions (–) diode ideality factor (–) number of moments to be calculate (–) number of TEM in parallel (–) number of TEM in series (–) Nusselt coefficients (–) power consumed by the fan (W) power generated by the PVT (W) power consumed by single TE module (W) Prandtl number (–) absolute value of electron’s charge (C) heat absorbed at the cold junction of TE module (W) heat dissipated at the hot junction of TE module (W) heat gain of the PVT system (W) thermal resistance of thermoelectric material (Ω) thermal resistance of heat pipe sink on exhaust air side (K/ W) Reynolds number (–) thermal resistance of heat pipe sink on fresh air side (K/W) parallel resistance of PV module (Ω) load resistance of PV panel (Ω) series resistance of PV module (Ω) total thermal resistance of heat pipe sink (K/W) distance between two fins of heat pipe fin (m) exhaust air outlet temperature of PVT-TEV system (°C) fresh air outlet temperature of PVT-TEV system (°C) fresh air outlet temperature of PVT system (°C) fresh air inlet temperature of TEV system (°C) voltage of PVT system (V) air velocity in the air duct (m/s) node volume (m3) average outdoor wind speed in winter (m/s) volume flow rate of air in the air passage (m3/h) maximum air velocity of air passing through the heat pipe sink fin (m/s) voltage at MPP (V) open circuit voltage of the PV module (V) working voltage of PVT system (V) diode thermal voltage (V) width of the PVT system (m) width of the TEV system (m) number of TE modules (–) the characteristic length of the air passage (m) number of fans (–) length of each region of the PVT system (m)

Greek symbols α δ δrg δsr Δp ΔQ ε ηfan ηfin ηh 1888

the absorption rate of the PV panel (–) fin thickness (m) thickness of the evaporation section of heat pipe (m) thickness of the condensing section of the heat pipe (m) frictional pressure drop of the air channel (Pa) heat loss through the air duct (W) black body emissivity (–) fan efficiency (–) fins efficiency (–) the thermal efficiency of PVT (–)

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ηPV λ λrg λsr μf ξ1 ρ τ

h hh i j in M out P PV STC TE TEV top bottom up low

the PVT electrical efficiency (–) thermal conductivity (W/(m K)) thermal conductivity of heat pipe materials (W/(m K)) thermal conductivity of heat pipe condensing section (W/ (m K)) dynamic viscosity of air (N s/m2) radiation heat transfer factor between the PV panel and the insulation board (–) density (kg/m3) the glass transmittance of the outer surface of the PV panel (–)

Subscripts and superscripts c cc e f

cold side of TE module cold side air outdoor environment air duct

hot side of TE module hot side air central node of regions in the air passage node area in the thickness direction of the insulation board indoor upper boundary node of regions in the air passage outlet of air passage insulation board PV panel standard testing condition thermoelectric module thermoelectric ventilator inner face of PV panel inner face of insulation board upper channel of the TEV lower channel of the TEV

performance of a photovoltaic/thermal (PV/T) air heating collector with condenser heat recovery to regenerate desiccant in a tropical climate. Results showed that the system can save about 18% of the total energy use [21]. Massimo et al. proposed and studied a PVT collector integrated with a thermal storage unit and a reverse cycle heat pump. The PVT collector was used to heat or cool fresh air by using daytime solar radiation and nighttime sky radiative cooling, respectively [22]. There are also some researches on PVT integrated thermoelectric cooling system. He et al. studied the performance of thermoelectric cooling and heating system driven by a heat pipe PV/T system. Results showed that the electrical efficiency of the heat pipe PV/T is about 16.7%, and the thermal efficiency of the system is about 23.5% [23]. Cheng et al. developed and investigated a solar-driven thermoelectric cooling module with a waste heat regeneration system. In the system, the PV provides electricity for thermoelectric cooling system, and the thermoelectric cooling system absorbs heat from the indoor room and then dissipates the heat to the cooling water [24]. In their studies on PVT system and thermoelectric cooling system, a number of data and analyses of great significance have been provided. In order to overcome the shortcomings of the traditional building shading and reduce the energy consumption of the building fresh air supply, this paper proposes and studies a photovoltaic thermal-compound thermoelectric ventilator (PVT-TEV) system. The system uses photovoltaic thermal collector (PVT) to generate electricity in winter while preheating the fresh air to achieve efficient use of solar energy. The preheated fresh air is further heated by the thermoelectric ventilator (TEV) system and then pumped into the indoor room. The system has the following advantages: (1) The PVT system preheats the fresh air, which can effectively reduce the fresh air energy consumption; (2) The TEV system can recover the indoor exhaust heat actively. By controlling the operating voltage input of the TEV system, the PVT-TEV system can actively adjust the temperature of fresh air; (3) The PVT system generates direct current which is stored in the battery to drive the TEV system; (4) The PVT-TEV system has the functions of the sunshade, power generation, waste heat recovery and fresh air supply for the building. The main objective of this work is to focus on the developing and modeling of a photovoltaic thermal-compound thermoelectric ventilator system. The model of PVT-TEV system was set up and validated by experimental data. The main parameters (including the operating current, indoor temperature and air volume flow rate) that affect the performance of PVT-TEV system are simulated and optimized by using the established model, which provides the foundation for the structural design and annual performance optimization of PVT-TEV system.

room by 70–90% [12], which indicates that the system can realize the aim of reducing buildings heat from the sun and saving energy in summer. However, due to the presence of external shading devices, the solar radiation cannot enter the building room in winter season, which is not beneficial for building heating and energy conservation [13,14]. In recent years, a new type of PV fixed exterior shading device which combines photovoltaic modules and building shade is widely used in buildings. The PV fixed exterior shading device can convert partial solar energy into electrical energy when functioning as a building shade [15]. However, when the PV fixed exterior shading device works in winter condition, the system just converts part of the solar energy into electrical energy and most solar energy dissipates in the form of heat into the outdoor environment. Compared with the traditional sun-shading method, the solar shading makes full use of solar energy while shading the sun. However, the solar energy is still not used effectively in winter. The performance of PV systems will be improved by decreasing the temperature of PV modules [16,17]. The PVT systems use PV cells to generate electricity and utilize the waste heat of PV system to heat indoor air or outdoor fresh air. They have two advantages: recovering the waste heat of PV panels and improving the PV electrical efficiency. The PVT systems are divided into liquid collectors and air collectors. The liquid PVT systems have a higher performance than the air collectors. However, the air collector systems have the advantage of a simple system and the air can be directly used for space heating. In recent years, many works have been done on the investigation of air PVT systems. Sohel developed a dynamic model of air-based photovoltaic thermal system. The simulation results show that the thermal efficiencies increased with the growth of ambient temperature [18]. Shahsavar and Ameri tested and analyzed an air-based PVT collector. Results showed that the system thermal efficiency is affected by the air mass flow rate, number of fans and setting glass cover on photovoltaic panels [19]. Agrawal and Tiwari studied a micro-channel photovoltaic thermal module (MCPVT) system [20]. Results show that the overall annual thermal gain and annual exergy gain of the MCPVT module were improved obviously compared with single channel photovoltaic thermal (SCPVT) system. When a PVT system is used for building heating, the heat gain of the PVT system is unstable because the solar radiation always changes with time during a day. It is difficult to meet the heating demands of a building by using the PVT system alone. Therefore, people often combine a PVT system with air conditioning system to achieve the purpose of heating buildings. There are some studies on PVT integrated air conditioner system in recent years. Sukamongkol et al. studied the

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2. Modeling of the PVT-TEV system

Table 1 Parameters and five extracted parameters from manufacturer under STC.

2.1. System description As shown in Fig. 1, the PVT-TEV system is mainly composed of the photovoltaic thermal (PVT) collector, the thermoelectric ventilator (TEV) and the airflow channel. The thermoelectric ventilator mainly includes the fresh air part, the thermoelectric cooling modules (TEM) part, and the exhaust air part. The thermoelectric cooling modules consist of P-type and N-type blocks of semiconductor materials. When direct current passes through semiconductor materials, the temperature of the cold side of TEM decrease and heat is absorbed from the surrounding environment. At the same time, the temperature of the hot side of TEM rise and heat is released outdoors according to Peltier effect. In summer, the PVT system installed above the building windows at a certain angle functions as an external shading device to block the solar radiation while using photovoltaic cells to generate electricity at the same time. When the PVT-TEV system works in winter, the fresh air is preheated by the PVT collector first, and then the fresh air is pumped into the hot side of the TEV system and further heated there before it is sent into the room. At the same time, the exhaust air is cooled by the heat sink exchangers on the other side of the thermoelectric modules when it is pumped from the indoor to the outdoor environment while the TEV system recovers the heat of the exhaust air to heat the fresh air based on Peltier effect. Therefore, the PVT-TEV system can make full use of the solar energy to preheat fresh air and recover waste heat from the exhaust air to further heat up the fresh air, achieving comprehensive utilization of solar energy and recovery of indoor exhaust heat. Moreover, the outlet temperature of the fresh air from the PVT-TEV system could be adjusted by changing the operating voltage of the thermoelectric modules.

Parameters from manufacturer

Five Extracted parameters under STC

Voc (V) Isc (A) Vm (V) Im (A) Ns

Iph,STC (A) Io,STC (A) Rs,STC (Ω) Rp,STC (Ω) Vt,STC (V)

38.7 9.34 31.4 8.6 60

I = Iph−I0 ⎡exp ⎛ ⎢ ⎝ ⎣ ⎜

U + IRs ⎞ ⎤ U + IRs −1 − Vt Rp ⎠ ⎥ ⎦

9.34245 8.58164 × 10−7 0.119755 457.051 2.38978

⎟

(1)

where Iph is the photo current (A); I0 is the diode saturation current (A); Vt = Nsn0KT/q is the diode thermal voltage (V); T is the temperature of the junction (K); Ns is the number of solar cells in series; n0 is the diode ideality factor; K is Boltzmann’s constant (1.380653 × 10−23 J/K); q is the absolute value of electron’s charge (1.60217646 × 10−19 Coulomb); Rp is the parallel resistance (Ω); Rs is the series resistance (Ω). The model was called five parameters model because there are five unknown parameters of Iph, I0, Rp, Rs and Vt, which can be extracted by the differential evolution (DE) method [26] with the data from the manufacturer. Table 1 shows the parameters from the manufacturer and five extracted parameters under standard testing condition (STC) (AM = 1.5, T = 25 °C, G = 1000 W/m2). Table 2 shows system parameters of the PVT-TEV system. The I–U equation under general conditions can be calculated by Eqs. (2)-(6) [27].

Iph =

G Iph, STC (1 + Kl (T −TSTC )) GSTC

(2)

3 Eg q ⎛ Eg T ⎞ I0 = I0, STC ⎛ exp ⎡ | − | ⎞⎤ ⎢ n 0 K ⎝ T STC T T ⎠ ⎥ ⎝ TSTC ⎠ ⎦ ⎣ ⎜

⎟

⎜

⎟

(3)

3

2.2. The electrical circuit model of PV module

T ⎞ ⎡ G ⎞⎤ 1−β ln ⎛ Rs = Rs, STC ⎛ ⎥ G STC ⎠ ⎦ ⎝ TSTC ⎠ ⎢ ⎝ ⎣ ⎜

The power generation device used in the PVT-TEV system is MonoSi photovoltaic cells. There are four options for the performance simulation of Mono-Si PV cells, including ideal single diode model, single diode Rs-model, single diode Rp-model and double diode model [25]. Considering the calculation speed and accuracy, the single diode Rpmodel (five-parameter method) is selected to calculate the performance of photovoltaic. The I–U characteristic of the PV module is as follows:

Rp = Rp, STC

Vt = Vt , STC

⎟

G GSTC

T TSTC

⎜

⎟

(4)

(5)

(6)

where Kl is the temperature coefficient of the short circuit current (V/ K); Eg = 1.121×[1–0.0002677(T − TSTC)] and β = 0.217.

Fig. 1. The schematic diagram of PVT-TEV system. 1890

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node area i of the air passage, the equation is:

Table 2 System parameters of the PVT-TEV system. Parameters

Values

Length of the PVT system, l Width of the PVT system, w Thickness of the PV panel, ePV Thickness of air duct, b Thickness of insulation board, eP Tilt angle of the collector Orientation of the collector Type of TE modules

1.640 m 0.992 m 0.008 m 0.050 m 0.040 m 30° South 9500/127/060 B (Number of elements N is 127) 0.330 m 0.242 m 0.170 m

Length of the TEV, lTEV Width of the TEV, wTEV Height of the upper channel of the TEV, HTEV,up Height of the lower channel of the TEV, HTEV,low The area of the single heat pipe

(ρVc ) f

(10) where T is the temperature (K); ρ is the density (kg/m3); V is the node volume, V = w × z × b (m3); c is the specific heat capacity (J/(kg K)); hci1 represents the convection coefficient between the air and the PV panel (W/(m2 K)); hci2 denotes the convection coefficient between the air and the insulation board in the air passage, (W/(m2 K)); Tf,i is the temperature of the node i of the air passage, which is the average temperature of the upper and lower boundary nodes, Tf,i = (Tf,M + Tf,M+1)/2 (K); Ai is the heat transfer area of the node region, Ai = w × z (m2); The subscript M and M + 1 are the upper and lower boundary nodes of regions in the air passage respectively; The subscripts f, PV, P are the air parameters in the air passage, the physical parameters of the PV panel and the insulation board respectively. Similarly, for the photovoltaic element of the node area i, the equation is:

0.135 m 0.193 m2

Based on Eq. (1), the I-U curves of the photovoltaic cells at any solar radiation and temperature of PV cells can be obtained. For a PV cell under actual operating conditions, the PV cell output current value is determined by the resistance value of the connected load. According to Ohm’s law, the current value can be calculated by the following formula:

VPV RPV

IPV =

dTf, i ̇ f (Tf, M + 1−Tf, M ) = hci1 Ai (TPV, i−Tf, i ) + hci2 Ai (TP, i−Tf, i )−mc dt

(ρVc )PV

dTPV, i = (1−ηPV ) ατAi G−hci1 Ai (TPV, i−Tf, i )−hco Ai (TPV, i−Te) dt 4 4 4 4 −ξ1 σAi (TPV, i−TP , i )−εPV σAi (TPV, i−Te )

−

λPV λ ePV w (TPV, i−TPV, i + 1)− PV ePV w (TPV, i−TPV, i − 1) z z

(7)

(11)

where RPV is the load resistance of PV cells, (7.5 Ω). In addition, if the PV system is under the control of Maximum Power Point Tracking (MTTP) [28], the output power can always be at its maximum. The power generation efficiency (ηPV) and thermal efficiency (ηh) of the PV panel can be calculated by the following equations:

For the node area i on the inside of the insulation board, the equation is:

ηPV = ηh =

(ρVc )P

̇ f (TPVO−Te) QPV mc = GAPV GAPV

dt

4 4 = hci2 Ai (Tf, i−TP, i ) + ξ1 σAi (TPV, i−TP, i )−

−

2 IPV RPV

PPV = GAPV GAPV

dTP,j i

λP eP w (TP, i−TP, i + 1) z 6

λP eP λ w (TP, i−TP, i − 1)− P Ai (TP,j i−TP,j +i 1)(j = 1) z 6 eP /6

(12)

(8)

For the node area i on the outside of the insulation board, the equation is:

(9)

(ρVc )P

where PPV is the power generated by the PVT system (W); IPV is the current of the PVT system (A); APV is the area of the PVT system (m2); QPV is the heat gain of the PVT system (W); TPVO is the temperature at the outlet of PVT air passage (°C); Te is the ambient temperature (°C); ṁ is the fresh air mass flow rate in the air passage, ṁ = Va × ρf (kg/s), where Va is the volume flow rate of air in the air passage (93 m3/s); ρf is the air density (kg/m3).

dTP,j i dt

= hco Ai (Te−TP, i ) + εP σAi (Te4−TP,4 i )− −

λP eP w (TP, i−TP, i + 1) z 6

λP eP λ w (TP, i−TP, i − 1)− P A ι (TP,j i−TP,j −i 1)(j = 6) z 6 eP /6

(13)

For the node area i on other layers of the insulation board, the equation is:

2.3. Numerical model of PVT system The thermal network model of the PVT system is shown in Fig. 2. The solar energy that the PVT system receives mainly includes two parts, a small part of which is converted into electric energy, and the rest is converted into heat. Then the PV panel distributes the heat to the outdoor environment, fresh air and the insulation board through convection and long-wave radiation. The physical model for calculating the heat transfer of PVT system is based on the regional model, which divides the PV panel, the air passage and the insulation board of the fresh air system into n regions along the air flow direction, taking n = 10, as well as ignores the temperature gradient within each separated region. A node is set in each area center, as shown in Fig. 2, assuming that the node can represent the physical parameters of the area and set the boundary node in the air channel for input boundary conditions for simplifying calculation. According to the principle of mass and energy balance, the following heat transfer equations of PVT system are established. For the

Fig. 2. Principle of heat transfer of the PVT system. 1891

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(ρVc )P

dTP,j i dt

=

λP eP λ e w (TP, i + 1−TP, i )− P P w (TP, i−TP, i − 1) z 6 z 6 λP λ j j−1 Ai (TP, i−TP, i )− P Ai (TP,j i−TP,j +i 1)(j = 2, 3, 4, 5) − eP /6 eP /6 (14)

In Eqs. (11)–(14), α is the absorption rate of the PV panel; τ is the glass transmittance of the outer surface of the PV panel; σ is the StephenBoltzmann constant (5.67 × 10−8 W/(m2 K4)); z is the longitudinal length of the node region, z = l/n (m); w is the lateral length of the node region, m; e represents thickness of the node region (m); ηPV represents the power generation efficiency of the PV panel; ξ1 is the radiation heat transfer factor between the PV panel and the insulation board; εPV is the black body emissivity of the PV panel (εPV = 0.94); εP is the black body emissivity of the insulation board (εP = 0.8); λ is thermal conductivity (W/(m K)); G is the radiation intensity incident on the surface of the solar panel (W/m2); superscript j represents the node area in the thickness direction of the insulation board. The finite difference method is used to solve Eqs. (10)–(14) in time and space. The relevant variables in the derivative form are replaced by finite difference quotient, and differential equations are transformed into difference equations at the time of Δt. The discrete equations at k and k + 1 moment can be got. Radiation heat transfer factor between the PV panel and the insulation board ξ1 can be obtained by the following equation [29]:

1 1 1 = + −1 ξ1 εPV εP

Fig. 3. Working principle of TEV.

ΔQ =

1 2 Qc = aITE (Tc + 273.15)− ITE r −k (Th−Tc ) 2

where va is the average outdoor wind speed in winter (2.34 m/s) [31]. The convection coefficient of the air with the PV panel as well as insulation board hci can be calculated by:

Q h = aITE (Th + 273.15) +

(17)

In terms of the 800 ≤ Re ≤ 7100,

Nubottom = 1.017Re 0.471 Pr 0.4

face

of

insulation

board,

when

Tc = Tcc−Qc R c

Xρf v μf

(for cold side)

Th = Thh + Q h Rh

(19)

where Pr is the Prandtl number; Re is the Reynolds number, which is calculated by:

Re =

(22) (23) (24)

where ITE is the operating current of thermoelectric modules, ITE = ITEV/Np (A); Tc is the cold side temperature of TE modules (°C); Th is the hot side temperature of TE modules (°C); a is the thermoelectric module’s Seebeck coefficient (V/K); r is the thermoelectric module's electrical resistance (Ω); and k is the thermoelectric module's thermal conductance (W/K). a, r and k are the temperature dependent parameters. The heat balance between the cold or hot side heat exchanger and the cooling or heating source can be given by:

(18) inner

1 2 ITE r −k (Th−Tc ) 2

2 PTE = aITE (Th−Tc ) + ITE r

where X is the characteristic length of the air passage, X = 2wb/(w + b) (m). There are many methods to calculate the Nusselt number. This paper chooses empirical formulas proposed by Candanedo [32], and the Nusselt number of PV panel and insulation board are calculated, respectively. In terms of the inner face of PV panel, when 250 ≤ Re ≤ 7500,

Nutop = 0.052Re 0.78 Pr 0.4

(21)

As shown in Fig. 3, the amount of heat absorbed at the cold junction of TE module (Qc), the heat dissipated at the hot junction of TE module (Qh), and the electric power consumed by TE module (PTE) are given by [34]:

(16)

Nuλ f X

1 ho d1

+

2.5. Numerical model of TEV system

The convection coefficient of the outside of PV panel and insulation board hco can be calculated by the following equation [30]:

hci =

1 d ln d1 2λP 0

where Tf is the temperature of fresh air (°C); Tin is the indoor temperature (°C); d1 is the insulation outer diameter of the air duct (0.221 m); d0 is the inner diameter of the air duct (0.165 m); lduct is the length of the air duct (1.5 m); ho is the convective heat transfer coefficient between indoor air and thermal insulation materials (10.5 W/m2 K); λP is the thermal conductivity of the insulation board (0.028 W/m K). d0, d1 and lduct are measured in the experiment, the values of other parameters refer to [33].

(15)

hco = 2.8 + 3.0va

πl duct (Tf −Tin )

(20)

where v is the velocity of air passing through the air duct, v = Va/ (w × b) (m/s); μf is the dynamic viscosity of air (1.86 × 10−5N s/m2).

(for hot side)

(25) (26)

Thh =

Tw + Toh 2

(27)

Tcc =

Tin + Toc 2

(28)

Thermoelectric material Seebeck coefficient where Rc is the thermal resistance of heat pipe sink on exhaust air side (°C/W); Rh is the thermal resistance of heat pipe sink on fresh air side (°C/W); Tcc and Thh are average temperature of exhaust air and fresh air respectively (°C); Toc and Toh are air temperature at the outlet of cold side and hot side air passages (°C); Tc is the cold side temperature of thermoelectric modules and Th is the hot side of temperature of thermoelectric modules (°C); Tw is the air temperature at the entrance of hot side air passage(°C). Thermoelectric material Seebeck coefficient can be calculated by [7]:

2.4. Numerical model of air duct The PVT system connects the TEV system through a circular air duct. When the fresh air from the PVT system passes through the air duct into the TEV system, there is a partial heat loss in the air duct. The heat loss through the air duct can be calculated by the following equation [33]: 1892

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a=

(α1 Th + α2 Th2/2 + α3 Th3/3 + α 4 Th4/4)−(α1 Tc + α2 Tc2/2 + α3 Tc3/3 + α 4 Tc4/4) Nnew × 71 Th−Tc

sink, and the last item is the convection heat transfer resistance of the heat pipe. The hif can be calculated by the following equations [35]:

(29)

α1 = 1.33450 ×

10−2

hif = 0.134

α2 = −5.37574 × 10−5

α4 = −1.27141 × 10−9 The resistivity of thermoelectric material:

×

Nnew 6 × 71 Inew (r1 Th + r2 Th2/2 + r3 Th3/3 + r4 Th4/4)−(r1 Tc + r2 Tc2/2 + r3 Tc3/3 + r4 Tc4 /4) Th−Tc

Afin = ηfin Atotal

where Atotal is the actual total area of the fins on both sides (m ); the fins efficiency ηfin is given by:

r1 = 2.08317 th ⎛ = ⎝

2hif

⎜

r2 = −1.98763 × 10−2

ηfin

r3 = 8.53832 × 10−5 10−8

Pfan =

10−1

(35)

(ṁ / ρf )Δp ηfan

COPh =

10−6

(36)

Q TEV xQ h = PTEV + yPfan xPTE + yPfan

(37)

where PTEV is the input electric power of TEV system (W); PTE is the input electric power of a single TE module (W); QTEV is the heat dissipated at the hot side of TEV system (W); Pfan is the power consumption of the fan (W); x is the number of TE modules (x = 12); y is the number of fans (y = 4). The total coefficient of performance (COPtotal) of the PVT-TEV is given by:

k3 = −8.64864 × 10−6 k 4 = 2.20869 × 10−8 where Nnew is the number of thermocouples (Nnew = 127); Inew is the maximum current (6 A). The values of these two parameters are provided by the manufacturer. Fig. 4 is the schematic diagram of heat pipe sink. The thermal resistance of the heat pipe sink can be calculated by the following equation [35]:

COPtotal =

̇ f (Toh−Te) QPVT − TEV mc = PTEV + yPfan xPTE + yPfan

where QPVT-TEV is the total heat gain of fresh air (W). Successful computer programming requires

δrg

1 1 1 δ 1 1 . + + + sr . + λrg Azf Azf hzf Ain hin λsr Aw Af hif

H

where ηfan is the fan efficiency (combination of electrical and mechanical efficiencies), which is assumed to be 0.5; Δp is the frictional pressure drop of air in the air channel (Pa), which is calculated based on the conventional duct sizing equations [37]. The coefficient of performance of the TEV in heating mode (COPh) is given by:

(31)

Rtotal =

2hif

The power consumed by fan Pfan is given by [36]:

N k = new 71 I × new 6 (k1 Th + k2 Th2/2 + k3 Th3/3 + k 4 Th4 /4)−(k1 Tc + k2 Tc2/2 + k3 Tc3/3 + k 4 Tc4 /4) Th−Tc

k2 = −3.89821 ×

⎞H ⎠ ⎟

λrg δ

λrg δ

Thermal conductivity:

k1 = 4.76218 ×

(34) 2

(30)

r4 = −9.03143 ×

(33)

where λf is the thermal conductivity of the air (0.02581 W/m K); cf is the specific heat capacity of air (kJ/kg K); δ is the fin thickness (0.002 m); s is the distance between two fins of heat pipe fin (0.004 m); do is the heat pipe diameter (0.005 m); H is the width of the fin (0.03 m). The effective heat transfer area of the fins Afin can be calculated by the following equation:

α3 = 7.42731 × 10−7

r=

λ f 0.681 1/3 s 0.2000 s 0.1134 ⎛ ⎞ Re Pr ⎛ ⎞ do ⎝H⎠ ⎝δ ⎠

(32)

where Rtotal is the total thermal resistance of heat pipe sink (K/W); δrg is the thickness of the evaporation section of heat pipe (0.002 m); λrg is the thermal conductivity of heat pipe materials (399 W/m K); Azf is the surface area of the inner wall of the heat pipe in the evaporation section (0.002283 m2); hzf is the heat transfer coefficient between the inner wall and the working fluid of the evaporation section (5800 W/m2 K); Aln is the condensing section surface area of the inner wall of the heat pipe (m2); hin is the heat transfer coefficient between the inner wall and the working fluid of the evaporation section (5800 W/m2 K); δsr is the thickness of the condensing section of the heat pipe (0.002 m); λsr is the thermal conductivity of heat pipe condensing section (236 W/m K); Aw is the heat pipe condensation section diameter area (m2); Af is the total surface area of the heat sink (0.19305 m2); hif is the heat transfer coefficient between air and heat sink (W/m2 K). The shape parameters are from experimental measurement and the values of other parameters refer to Ref. [34]. For a given heat pipe sink, the first 5 items of Eq. (32) can be directly calculated according to the structural parameters of the heat pipe

(38) a

Fig. 4. The schematic diagram of heat pipe sink. 1893

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Fig. 7. Whole day temperature profile of Tin, Tout and solar radiation.

Fig. 5. The simulation flowchart.

understanding of the input data, data flow, and output data in each submodel. Fig. 5 is a flow chart of the PVT-TEV system simulation process. The program first calculates the parameters of the PVT part, and then subtracts the fresh air outlet temperature Tf,n minus the temperature drop of the air duct as input to the TEV part.

Fig. 8. The comparison of estimated and measured electrical efficiency and electrical power.

the TEV system. It is necessary to explain that the electrical power of PVT is consumed by an electrical resistance, and the photovoltaic system is not in MTTP control condition. The input parameters in the simulation are solar radiation, outdoor temperature and indoor temperatures. The actual meteorological data in the experimental test are selected as input conditions, as shown in Fig. 7. Comparison of simulation and experimental parameters used in this section mainly include the power generation performance parameters, the PVT heating characteristic parameters and the TEV heating characteristic parameters, as shown in Figs. 8–11. Fig. 7 shows the solar radiation, the indoor temperatures, and the outdoor temperature varying with time during the test day. It can be observed from Fig. 8

3. Model verification The PVT-TEV system model needs to be verified by using experimental data. As shown in Fig. 6, PVT-TEV test platform was set up in Changsha area of China [38], and one day’s data were selected to compare the results with the simulated results in this section. The PVTTEV system structure parameters are shown in Table 1. The size of the PVT system is 1.640 m × 0.992 m × 0.100 m, and the PVT collector installation angle is 30°. The type of thermoelectric module is 9500/ 127/060 B and 12 thermoelectric modules (Ns = 4, Np = 3) are used in

(b) TEV system (before the fiberglass was insulated)

(a) PVT system

Fig. 6. Schematic diagram of the experiment system. 1894

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Fig. 12. Whole day temperature profile of Tin, Tout and solar radiation for modeling.

Fig. 9. The comparison of estimated and measured fresh air outlet temperature.

Fig. 10. The comparison of estimated and measured heat gain and thermal efficiency of PVT system. Fig. 13. Electrical efficiency of PVT under different fresh air volume flow rate.

working current. In order to study and analyze the impact of various factors on the performance of PVT-TEV system, the system model and simulation method established in the third section are used to simulate and analyze the system performance. When simulating the system, the photovoltaic system is assumed to be working under the control of MTTP, that is, the PV system is always working at the maximum electrical efficiency point. 4.1. Influence of fresh air mass flow rate on the performance of PVT-TEV system When the PVT-TEV system works, the outdoor fresh air is preheated by the PVT system and then heated further by the TEV system before it is pumped into the room under the pressure of the fan. The fresh air volume flow rate not only affects the electrical power generation and thermal characteristics of the PVT system but also affects the thermal performance of the TEV system. In order to study the influence of the fresh air volume flow rate on the performance of PVT-TEV system, the performance of the PVT-TEV system in the fresh air volume flow rate of 63 m3/h, 93 m3/h, 123 m3/h and 153 m3/h is simulated, respectively. When the system is simulated, the working current of TEV is kept at 2 A. The solar radiation, indoor temperature, and outdoor temperature are shown in Fig. 12. Fig. 13 is the electrical efficiency of the PVT system under different fresh air volume flow rate. It can be seen that the electrical efficiency of the PVT system gradually increases after the photovoltaic panel starts to work, and reaches the maximum value at about 9:45 under different fresh air volume flow rate. Subsequently, affected by the solar radiation and the temperature of the photovoltaic panel, the electrical efficiency

Fig. 11. The comparison of estimated and measured COP of TEV and PVT-TEV.

that the simulated values of electrical power and electrical efficiency are larger than the experimental values most of the time. This is because the PV cells have been used for nearly one year before the test, resulting in a certain decline in the performance of the photovoltaic cells. From Figs. 8–11, it can be seen that the model simulation results established in this paper are in good agreement with the experimental results in general, indicating that the PVT-TEV model established in this paper has higher accuracy. 4. Results and analysis The performance of PVT-TEV system is affected by many factors, such as solar radiation, ambient temperature, indoor temperature and 1895

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Fig. 14. Heat gain of PVT under different fresh air volume flow rate.

Fig. 16. Heat gain of TEV system under different fresh air volume flow rate.

Fig. 15. Thermal efficiency of PVT under different fresh air volume flow rate.

Fig. 17. COP of TEV under different fresh air volume flow rate.

of PVT gradually decreases. The greater the fresh air volume flow rate is, the greater the electrical efficiency of PVT is. At 13:00, the electrical efficiency is 14.98% under fresh air volume flow rate of 63 m3/ h. When the electrical efficiency of PVT increases to 153 m3/h, the PV electrical efficiency is 15.48%, because the larger the fresh air volume flow rate is, the lower the temperature of the photovoltaic panel are, and thus the higher the power generation efficiency is. In addition, it can be seen that the electrical efficiency is less affected by fresh air volume flow rate in the morning and the evening, because the heat gain of PVT is much smaller than that at noon, and the PVT can achieve better cooling effect when the fresh air volume is 63 m3/h in the morning and evening. Figs. 14 and 15 are the heat gain and thermal efficiency of PVT under different fresh air volume flow rates, respectively. Under the influence of solar radiation, the heat gain increases gradually after the PVT system begins to work, reaching the maximum at 12:30 and then gradually decreasing. It can be seen that the heat gain and thermal efficiency of PVT system rise with the increase of fresh air volume flow rate. This is because a larger fresh air flow rate can lead to lower PV panel temperature, thus less heat is dissipated to the environment through the PV panel and the insulation board. It can be observed that the thermal efficiency of the PVT system is relatively small when the system starts to work. This is because the photovoltaic panel and insulation materials have a certain heat capacity and the solar energy is absorbed by the photovoltaic panel and insulation materials during a period after the system works in the morning. In addition, the thermal efficiency increases sharply after 16:30. This is because there is a certain amount of heat storage in the PV panel and thermal insulation materials. When the solar radiation continues to decline after 16:30, the heat is released to heat the fresh air, increasing the system thermal efficiency dramatically.

Figs. 16 and 17 are the heat gain and the COP of TEV under different fresh air mass flow rate respectively. It can be seen that the heat gain and COP of TEV system gradually decrease after the system works, reach the minimum at about 12:20, and then gradually increase. This is because the temperature of the fresh air entering the hot side of the TEV increases with the growth of the heat gain of the PVT (the trend of the heating gain of the PVT is shown in Fig. 12). A higher temperature of fresh air entering the TEV leads to a larger temperature difference between the hot and cold sides of the thermoelectric modules. Thus, the heat gain and COP of the TEV system become smaller. It can also be

Fig. 18. Fresh air outlet temperature of PVT-TEV under different fresh air volume flow rate. 1896

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Fig. 19. COP of PVT-TEV under different fresh air volume flow rate.

Fig. 21. Fresh air outlet temperature of PVT-TEV system under different working current.

and 153 m3/h, and the COP of the PVT-TEV system reaches the maximum value on the test day with 17.2, 17.9, 15.3 and 11.7 respectively. In addition, it can be observed that the COP under the fresh airflow rate of 63 m3/h is larger than the fresh airflow rate of 93 m3/h in the morning and afternoon. However, when the solar radiation becomes larger at noon, the COP of PVT-TEV system under fresh airflow rate of 93 m3/h becomes larger than the fresh airflow rate of 63 m3/h. This is because when the solar radiation is large, increasing the fresh airflow rate can significantly increase the heat gain of fresh air in PVT. However, when the solar radiation is small, increasing the fresh airflow rate is not obvious for improving the PVT performance. 4.2. Influence of current on the performance of PVT-TEV system The heat gain of the PVT-TEV system is divided into two parts, one is the heat gain of the PVT system and the other is the heat gain of the TEV system. The heat gain of the PVT system is not affected by working current, thus the heat gain difference of PVT-TEV system under different working current is caused by the TEV system. Fig. 20 is the heat gain of PVT-TEV system under different working current. It can be seen that the higher the working current is, the greater the heat gain of the TEV system is and the greater the total heat gain of the PVT-TEV system is. Fig. 21 is the fresh air outlet temperature of the PVT-TEV system under different working current. It can be seen that the fresh air temperature rises as solar radiation increases gradually after the system begins to work. The fresh air temperature reaches its maximum at 13:20, and then decreases with decreasing solar irradiance. It can be observed that the higher the current is, the higher the outlet

Fig. 20. Heat gain of PVT-TEV system under different working current.

seen that the larger the fresh air volume flow rate is, the more heat gain of the TEV system is. This is because the fresh air velocity increases with the rise of fresh air volume flow rate and the heat resistance of heat pipe becomes smaller, thus the thermoelectric module’s heat dissipation conditions and heating performance are improved. However, because of the increase of fan power when the fresh air volume flow rate increases, the COP of TEV system under fresh air volume flow rate of 123 m3/h and 153 m3/h is smaller than that under 63 m3/h and 93 m3/h. Fig. 18 is the fresh air outlet temperature of PVT-TEV system under different fresh air volume flow rate. It can be seen that the greater the fresh air volume flow rate is, the lower the fresh air outlet temperature is. Affected by the solar radiation intensity, the temperature of the fresh air gradually increases after the PVT-TEV system starts to work and reaches its maximum at about 13:20 in the afternoon, and then decreases gradually. Fig. 19 is the COP of the PVT-TEV system under different fresh air volume flow rates. The COP of the PVT-TEV system is affected by the system's heat gain and input power. The input power of the PVT-TEV system includes the power of TE modules and the four fans. When the system works under different fresh airflow rates, the power of the TE modules remains the same, but the greater the fresh air volume flow rate is, the greater the four fans power is. The fans power is 6.24 W when the fresh airflow rate is 93 m3/h under real working condition. According to Eq. (37), the fans power of PVT-TEV system is 1.94 W, 14.4 W and 27.8 W when the fresh airflow rate is 63 m3/h, 123 m3/h and 153 m3/h respectively. It can be observed that when the fresh airflow rate increases from 93 m3/h to 123 m3/h and 153 m3/h, the system's heating gain increases, but the system COP decreases due to the increase of the input power. At 12:30, the heating gain of the PVT-TEV system is 339 W, 410 W, 462 W and 504 W when the fresh airflow rate is 63 m3/h, 93 m3/h, 123 m3/h

Fig. 22. COP of TEV system under different working current. 1897

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Fig. 23. COP of PVT-TEV system under different working current.

Fig. 25. Fresh air outlet temperature of PVT-TEV system under different exhaust air temperature.

Fig. 24. Heat gain of PVT-TEV system under different exhaust air temperature. Fig. 26. COP of PVT-TEV system under different exhaust air temperature.

temperature of the fresh air is. The fresh air outlet temperature at 2 A, 3 A, 4 A, 5 A at 13:20 noon is 27.9 °C, 29.4 °C, 31.2 °C and 33.0 °C respectively. Fig. 22 shows the COP of TEV system under different current. It can be observed that the COP decreases with the increase of the working current of TEV under the working current of 2 A, 3 A, 4 A and 5 A. After the TEV system starts to work, the COP of the TEV system gradually decreases and reaches a minimum around 13:20 and then gradually increases. This is because the heat gain of the PVT increases gradually with the increase of solar radiation, and reaches the maximum value at 13:20, and then decreases gradually, resulting in the same trend of fresh air outlet temperature changes of the PVT system. The temperature difference of the cold and hot sides of the thermoelectric modules increases with the increase of the fresh air inlet temperature. For thermoelectric cooling modules, the COP decreases with the increase of the temperature difference of the cold and hot sides of the thermoelectric modules, resulting in a decrease of the COP of the TEV system. Fig. 23 shows the COP of PVT-TEV system under different current. The smaller the current is, the greater the COP of PVT-TEV system is under the current of 2–5 A. When the current is 2 A, the maximum COP is 17.9 and the minimum COP is 8.4, while the maximum COP of the system is only 5.3 and the minimum COP is 3.3 when the current is 5 A.

show the heat gain and the fresh air outlet temperature of the PVT-TEV system under different indoor air temperature. Fig. 26 is the COP of the PVT-TEV system under different indoor temperature. It can be observed that the heat gain of PVT-TEV system, the fresh air outlet temperature and the COP of the PVT-TEV system grow with the increase of the indoor temperature. When the indoor room air temperature is 18 °C at 12:00, the heat gain of the PVT-TEV system is 383.4 W, the fresh air outlet temperature is 24.7 °C and the COP of the PVT-TEV system is 16.5. When the indoor air temperature increases to 24 °C, the heat gain of the PVT-TEV system is 411.1 W, the fresh air outlet temperature is 25.6 °C and the COP of the PVT-TEV system is 18.7 relatively. This is because a higher indoor air temperature leads to a lower temperature difference between the cold and hot sides of the TE modules and a lower electrical power consumed by TE modules. Thus, the performance of the thermoelectric modules becomes better. 5. Discussion According to Fig. 17, it can be calculated that the average electrical efficiency of PVT system is 12.03%, 13.15%, 13.24% and 13.31% respectively and the average COP of the PVT-TEV system is 13.80, 13.85, 11.42, and 8.54 respectively when the fresh air volume flow rate is 63 m3/h, 93 m3/h, 123 m3/h and 153 m3/h. Compared with conventional split-type air conditioner with COP of 2.4 in average, earth-air heat exchanger fresh air system with COP of 2.11 in the winter [39] and the thermoelectric ventilator with the average COP of 2.6 [40], the performance of the PVT-TEV system is more efficient. The performance of the PVT-TEV system can be improved further by selecting TE and PV

4.3. Influence of exhaust air temperature on the performance of PVT-TEV system In order to study the effect of exhaust air temperature on the performance of PVT-TEV, the performance of the PVT-TEV system is simulated when the system operating current is 2 A and the indoor temperature is 18 °C, 20 °C, 22 °C and 24 °C respectively. Figs. 24 and 25 1898

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References

system with higher performances. As the efficiency of the available commercial PV is about 15–20%, there is still a large room for the PV products to improve, considering the maximum PV efficiency of 39% [41]. Meanwhile, the TE modules used in the present researches have a thermoelectric figure of merit (ZT) of about 0.6–0.7, which is not high considering the progress of TE technology. However, the ZT of the TE materials of the latest development can be as high as 2.4 at 300 K [42]. The heating capacity and the fresh air outlet temperature of the PVT-TEV system can be adjusted by changing the operating currents of the TE modules. It can be seen from the simulation results that the larger the current operation, the lower the COP of the system. In actual operation, the number of the thermoelectric modules can be increased so that high system performance can be obtained to achieve larger PVTTEV heating capacity. The PVT-TEV system uses the PVT power generation to drive the TEV system. According to Figs. 10 and 11, it can be calculated that when the fresh air volume is 93 m3/h, the average generating power of the working day is 119.6 W, and the operating current of the TEV system is 2 A, and the total average power consumption of thermoelectric modules and the fans of the TEV system is about 27.5 W. Therefore, the photovoltaic system can meet the electricity demand of the TEV system in the simulation day. In actual operation, it can convert alternating current to direct current to drive the PVT-TEV system when the power generated by the PVT system is not enough for the TEV system on cloudy or rainy days.

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6. Conclusion In this paper, a PVT-TEV model is proposed and validated with an experimental system under real weather conditions. Results show that the experimental values are in good agreement with the simulation results. With the model established in this paper, the performance of the PVT-TEV system under changing working conditions is also analyzed. The results show that the fresh air volume flow rate has a significant effect on the performance of the PVT-TEV system. Increase in the fresh air volume flow rate can improve the electrical efficiency of the PVT system and increase the PVT-TEV’s heat gain. Under the working current of 2 A, the average electrical efficiency of the PVT system is 12.03%, 13.15%, 13.24% and 13.31% respectively when the fresh air volume flow rate is 63 m3/h, 93 m3/h, 123 m3/h and 153 m3/h. However, due to the increase of fan power, the COP of the PVT-TEV system does not always increase with the increase of fresh air volume flow rate. The COP values of the PVT-TEV system working under the air volume of 63 m3/h and 93 m3/h respectively are close enough, with the average of 13.80 and 13.85 respectively, but the system's heat gain is greater at 93 m3/h than at 63 m3/h. When the air volume reaches 123 m3/h and 153 m3/h, the COP of the system gradually decreases to 11.42 and 8.54 respectively. The working current of the TE modules and indoor temperature have no effect on the performance of the PVT system, but they have a significant influence on the performance of the TEV system. The heat gain of the TEV system and the fresh air outlet temperature of the PVTTEV increase with the increase of the working current. When the system operates under currents of 2 A, 3 A, 4 A and 5 A respectively, the COP of the PVT-TEV system decreases with the increase of the working current. The higher the indoor temperature, the higher the heat gain of the PVTTEV system and the higher the COP of PVT-TEV system. Acknowledgements The work described in this paper is sponsored by the National Natural Science Foundation of China (Grant Number: No. 51708194, No. 51578221 and No. 5150714), and the National Natural Science Foundation of Hunan province China (Grant Number: No. 2015JJ4002). 1899

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